A self-proclaimed leader of the pro-Russian separatists in eastern Ukraine has pledged to take control of cities controlled by Ukraine's government.

Aleksandr Zakharchenko said on May 29 in the rebel-controlled Ukrainian city of Donetsk that separatist forces will regain control over Slovyansk, Kostyantynivka, and Krasnoarmiyisk, which were briefly held by the rebels last year.

He said "as soon as we realize" that retaking the towns by political means is not possible, "a decision will be made to liberate these territories."

Zakharchenko, who calls himself the head of state and prime minister of the self-styled Donetsk People's Republic, has previously pledged that rebel military forces would retake areas in Ukraine's Donetsk region not currently held by the separatists.

More than 6,100 people have died in fighting between Ukrainian forces and the Russian-backed separatists.